That’s what keeps me from truly believing in many conspiracy theories - they would have to be too big and too many people would have to know, etc...
but - I do think it’s fair to wonder.
One of the best explanations I’ve read is by the great Bill Whittle, in his essay, “Seeing The Unseen, pt. 2” at his website, ejectejecteject.com. His website got overhauled last year and most of his essays got deleted, but it’s still available at the web archive:
http://web.archive.org/web/20070509072531/http://www.ejectejecteject.com/
Scroll down to the entry for April 8, 2007.
Bill’s one of my favorite writers/commentators now, I highly recommend this essay in particular. It’s kinda long, but it’s worth the read.
The problem is, we have seen government able to keep secrets in the past. That is what makes conspiracies believable.
The experiment on 400 black men with Syphillis in Tuskegee went on for 40 years until 1972. Too big? Too many people? That story was hidden for over 25 years until 1997.
We only seem to learn of these government conspiracies 25 to 50 years after the fact. That is why you have people believing we never landed on the moon.
It is patently obvious to me that the New York City Massacre and destruction of the World Trade center on 9/11/2001 was an outside job, but many people distrust their government to tell the the truth, and it leads to suspicion, even where there is no conspiracy.